That’s a neat tachometer! Where did you get your hands on this??? Its by Smiths, so Brit most prolly.
When you have the wiring diagram with you (a real blessing) why do you need directions??? How does it not list where to connect the remaining two wires?? Please scan the diagram and post a high resolution copy here, it would be easy to follow for sure. I cant make much out in the photograph.
I don’t know how Storm has deciphered the color codes for the wires but I must warn you that you have to be very sure before you wire up this instrument (or any electric device in an auto for that matter) and energize it. You might end up doing permanent damage to the tacho.
The problem is color codes are not standard, the Brits may consider a particular color to designate live for example while maybe the Japs consider it something else. So tread carefully!
I have a tacho myself which also has four wires but they are black, green, red and white. I won’t tell you how the recommended wiring diagram is for my meter because I don’t want you to blindly follow it. I would however say that Storm’s key may be correct.
Would wait till you post a good copy of the instruction sheet.